In future wireless networks, a large variety of services, such as voice/video/data/image, are expected. The most precious resource in most wireless systems is the radio spectrum. To maximize its effective use, packet switched wireless access using code division multiple access (CDMA) has been pursued and offers increased service quality and transmission bandwidth. These CDMA systems provide reduced multiple path distortion and co-channel interference, and avoid the need for frequency planning that is common with frequency division multiple access (FDMA) and time division multiple access (TDMA) systems.
In a CDMA system, a unique binary spreading sequence (a code) is assigned for each call to each user. Multiplied by the assigned code, the user's signal is "spread" onto a channel bandwidth much wider than the user signal bandwidth. The ratio of the system channel bandwidth to the user's bandwidth is commonly called "the spreading gain." All active users share the same system channel bandwidth frequency spectrum at the same time. Given a required signal-to-interference (S/I), the equivalent system capacity is proportional to the spreading gain. The signal of each user is separated from the others at the receiver by using a correlator keyed with the associated code sequence to "de-spread" the desired signal.
In these CDMA systems, there is a continuing need to increase the performance of the system by accommodating users having different source rates.